To separate fog or liquid particles from the exhaust gases of chemical processes, it is known to use so-called tubular electrostatic precipitators, which comprise a bundle of tubes or ducts, which are similar in cross-section and define parallel flow paths for the gas. In each tube or duct, a taut wire is concentrically disposed. This wire is connected to one terminal of a source of a high d.c. voltage whose other terminal is connected to the walls of the flow path. In the operation of the precipitator, the electrostatic field which effects the separation is established between the wire and the surrounding surface of the tube or duct.
It is also known to use synthetic-resin material tubes in electrostatic precipitators for separating corrosive particles of fog or liquid. While tubes of synthetic-resin material have a high resistance to corrosion, they cannot be used at elevated temperatures. For example, it is not possible to use tubes of PVC-S at temperatures above 60.degree. C., tubes of polypropylene and composite tubes of PVC and glass fiber-reinforced plastics material at temperatures above about 75.degree. C., and tubes of PVC-HT at temperatures above about 80.degree. C.
With tubes made of PVC-S and PVC-HT, the relaxation tendencies must also be considered since even at temperatures below those mentioned above, significant deformation may occur so that the tolerances regarding dimensions and disposition can no longer be adhered to. These tolerances are very small in order to ensure optimum electrostatic conditions. Failure to adhere to the tolerances considerably detracts from the usefulness of the precipitator and can possibly result in failure thereof.
While the relaxation can be fairly well controlled with composite tubes consisting of thermoplastics and of synthetic-resin material reinforced with glass fibers, the use of such tubes involves other difficulties. For example, different coefficients of expansion give rise to considerable stress in the composite and lead to cracking thereof. Through these cracks, any corrosive liquid which has been precipitated can penetrate the composite and destroy the support structure. Because the structural elements of the electrostatic precipitator are relatively large, the required dimensional tolerances often can be adhered to only with great difficulty in view of the properties of the material and the technology used to process the same.
Most synthetic-resin materials are also very good electrical insulators. While this property is highly desirable in many cases in which synthetic-resin materials are used, it is entirely undesirable in electrostatic precipitators. Although the collector electrodes need not be electrically conducting when it is desired to separate fog and liquid particles, because the film of moisture deposited on said collector electrodes constitutes on the surface of the inherently insulating material a layer having a sufficiently high electrical conductivity, this is only a "pseudo-conductivity", depending upon the formation of a continuous liquid film, i.e. on a correspondingly high moisture content of the flowing gas at all times.
Those synthetic-resin elements which consist of weldable thermoplastics are joined by welding.
Synthetic-resin elements consisting of composite material, such as PVC and glass fiber-reinforced synthetic resins, are sometimes joined by the synthetic resin, which is applied in a liquid state to the thermoplastic material, and partly by the reinforcing materials, such as glass fiber mats, which are embedded in the synthetic resin. These manufacturing operations are performed almost exclusively by hand and are highly expensive. Experience has shown that these techniques do not always result in a homogeneous structure of the material.